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SubjectRe: iov_iter_pipe warning.
On Mon, Sep 11, 2017 at 09:07:13PM +0100, Al Viro wrote:
> Strictly speaking that behaviour doesn't violate POSIX. It is, however,
> atrocious from the QoI standpoint, and for no good reason whatsoever.
> It's quite easy to do better, and doing so would've eliminated the problems
> in pipe-backed case as well (see below). In addition to that, I would
> consider teaching bio_iov_iter_get_pages() to take the maximal bio size
> as an explict argument. That would've killed the need of copying the
> iterator and calling iov_iter_advance() in iomap_dio_actor() at all.
> Anyway, the minimal candidate fix follows; it won't do anything about
> the WARN_ON() in there, seeing that those are deliberate "program is
> doing something bogus" things, but it should eliminate all crap with
> ->splice_read() misreporting the amount of data it has copied.

... and after minimal testing and fixing a braino in "found an IO error"
case, that's

diff --git a/fs/iomap.c b/fs/iomap.c
index 269b24a01f32..012e1f247e13 100644
--- a/fs/iomap.c
+++ b/fs/iomap.c
@@ -832,6 +832,7 @@ iomap_dio_actor(struct inode *inode, loff_t pos, loff_t length,
struct bio *bio;
bool need_zeroout = false;
int nr_pages, ret;
+ size_t copied = 0;

if ((pos | length | align) & ((1 << blkbits) - 1))
return -EINVAL;
@@ -843,7 +844,7 @@ iomap_dio_actor(struct inode *inode, loff_t pos, loff_t length,
/*FALLTHRU*/
case IOMAP_UNWRITTEN:
if (!(dio->flags & IOMAP_DIO_WRITE)) {
- iov_iter_zero(length, dio->submit.iter);
+ length = iov_iter_zero(length, dio->submit.iter);
dio->size += length;
return length;
}
@@ -880,8 +881,11 @@ iomap_dio_actor(struct inode *inode, loff_t pos, loff_t length,
}

do {
- if (dio->error)
+ size_t n;
+ if (dio->error) {
+ iov_iter_revert(dio->submit.iter, copied);
return 0;
+ }

bio = bio_alloc(GFP_KERNEL, nr_pages);
bio_set_dev(bio, iomap->bdev);
@@ -894,20 +898,24 @@ iomap_dio_actor(struct inode *inode, loff_t pos, loff_t length,
ret = bio_iov_iter_get_pages(bio, &iter);
if (unlikely(ret)) {
bio_put(bio);
- return ret;
+ return copied ? copied : ret;
}

+ n = bio->bi_iter.bi_size;
if (dio->flags & IOMAP_DIO_WRITE) {
bio_set_op_attrs(bio, REQ_OP_WRITE, REQ_SYNC | REQ_IDLE);
- task_io_account_write(bio->bi_iter.bi_size);
+ task_io_account_write(n);
} else {
bio_set_op_attrs(bio, REQ_OP_READ, 0);
if (dio->flags & IOMAP_DIO_DIRTY)
bio_set_pages_dirty(bio);
}

- dio->size += bio->bi_iter.bi_size;
- pos += bio->bi_iter.bi_size;
+ iov_iter_advance(dio->submit.iter, n);
+
+ dio->size += n;
+ pos += n;
+ copied += n;

nr_pages = iov_iter_npages(&iter, BIO_MAX_PAGES);

@@ -923,9 +931,7 @@ iomap_dio_actor(struct inode *inode, loff_t pos, loff_t length,
if (pad)
iomap_dio_zero(dio, iomap, pos, fs_block_size - pad);
}
-
- iov_iter_advance(dio->submit.iter, length);
- return length;
+ return copied;
}

ssize_t
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